TPR Orgo, carbocation rearrangement? (Chapter 5)

[Padfoot]

On page 167, it says that one of the carbocations cannot rearrange from a secondary carbocation to a more stable tertiary carbocation. I don't know how to upload a pic, but its a 3,3-dimethyl heptane with a positive charge on C-6. Why can't the positive charge just hydride shift until it reaches the 4th carbon, and then a methide shift from one of C-3's methyls can shift the positive charge to C-3, creating a more stable tertiary carbocation?


Any help would be greatly appreciated!

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[Czarcasm]

On page 167, it says that one of the carbocations cannot rearrange from a secondary carbocation to a more stable tertiary carbocation. I don't know how to upload a pic, but its a 3,3-dimethyl heptane with a positive charge on C-6. Why can't the positive charge just hydride shift until it reaches the 4th carbon, and then a methide shift from one of C-3's methyls can shift the positive charge to C-3, creating a more stable tertiary carbocation?


Any help would be greatly appreciated!

Because it's unfavorable and therefore an energy costly process, which is why they can only occur between neighboring carbons. It's just a general rule of organic chemistry to file away .

 

[DrknoSDN]

To add a little to what Czarcasm said, if the positive charge is on C6 you could shift a hydrogen from C5 to C6 to form the C5 secondary carbocation easily but after that there is no real driving force to continue shifting hydrogens.

If you think about it as a multi-step process, each step needs to be at least slightly favorable energetically to occur spontaneously. So once you had a C5 secondary carbocation there is not enough of a favorable energy shift (C5 secondary becoming C4 secondary) to overcome the energy barrier required to move the C4 hydrogen to C5.

Alternatively if it was 3,3-dimethyl heptane with C5 being a primary carbocation it could shift to C4 becoming a secondary and a methyl shift could occur to form the tertiary C3 carbocation. In that situation both steps are favorable so you can have a hydrogen shift followed by a methyl shift occur spontaneous. Albeit you would probably get multiple products and varying yields for each.

Note:: I continued using C5 and C4 nomenclature even tho if the original product was 3,3-dimethyl heptane the numbering would reverse so it would technically be C1 and C2 carbocations at that point... No need for corrections. =)

 

[Concrete Jungle]

Isn't there just an energy barrier to overcome from going from one secondary carbocation to another? If so, the probability that it could happen is just a function of temperature right? Seems kinda weird to say it "can't" undergo rearrangement.

 

[Padfoot]

To add a little to what Czarcasm said, if the positive charge is on C6 you could shift a hydrogen from C5 to C6 to form the C5 secondary carbocation easily but after that there is no real driving force to continue shifting hydrogens.

If you think about it as a multi-step process, each step needs to be at least slightly favorable energetically to occur spontaneously. So once you had a C5 secondary carbocation there is not enough of a favorable energy shift (C5 secondary becoming C4 secondary) to overcome the energy barrier required to move the C4 hydrogen to C5.

Alternatively if it was 3,3-dimethyl heptane with C5 being a primary carbocation it could shift to C4 becoming a secondary and a methyl shift could occur to form the tertiary C3 carbocation. In that situation both steps are favorable so you can have a hydrogen shift followed by a methyl shift occur spontaneous. Albeit you would probably get multiple products and varying yields for each.

Note:: I continued using C5 and C4 nomenclature even tho if the original product was 3,3-dimethyl heptane the numbering would reverse so it would technically be C1 and C2 carbocations at that point... No need for corrections. =)

Thanks, your explanation was very helpful